Same for me, I wrote "Co nejdřív děláš" and it was not accepted. Any reason ?
Is the personal pronoun (ty /vy) compulsory here? My answer "Co nejdřív děláte?" was marked as wrong and the suggestion was to add "vy" at the end. Maybe it has to do with word order here?
For what it's worth, my answer -- Co děláš nejdřív -- was accepted, without the pronoun.
I guess (ale nevím) that it is not ok to have the verb last. Usually, the end of the sentence is under stress, and it seems to be not possible to stress "do" here - which kind of makes sense, if I think about it. Either one may stress FIRST or YOU.
Can this question be used to mean "What do I/does one do first?" as it does in English?
Can you propose a context?